This invention relates to improvements in thread-forming fasteners and to thread-rolling dies used for making the same.
One type of thread-forming screw which has been commercially successful in many parts of the world is that of the type shown in the U.S. Pat. No. 3,195,196 to Phipard that issued July 20, 1965. The screw shown in that patent is of a type having a lobular pitch surface cross section in the form of a generally arcuate polygon with an odd number of arcuate sides merging gradually with intermediate thread-swaging lobes. The work-entering end of the screw is tapered toward the work-entering point of the screw for insertion into the workpiece pilot hole. Preferably the screw has three lobes. A screw of the foregoing type has a generally uniform lobulation throughout its threaded length. Since the work-entering end forms the thread and the holding or shank section subsequently mates with the thread in the workpiece, the thread must have a lobulation that provides a low driving torque for thread formation and yet a high locking ability for engagement with the workpiece. Consequently, the amount of lobulation in the screw is necessarily a compromise to effect the best possible combination of low-driving torque and good locking ability. Generally, the screws are designed to favor thread forming.
It has also been proposed to produce thread-forming fasteners with a lobular work-entering portion and a circular holding section. Typical of such screws are those shown in U.S. Pat. Nos. 3,246,556 to Phipard that issued Apr. 19, 1966 and to Muenchinger 3,681,963 that issued Aug. 8, 1972. However, where the holding section is of circular cross section there is a reduction of stripping torque and a reduction in the effective locking action with the workpiece as compared to the lobular form. Because the holding section is of circular cross section there are no lobes or like areas that tend to bite into the workpiece to effect a locking action and an increase in both strip torque and locking action.
Another problem in self-tapping fasteners resides in the difficulty in aligning the fastener with the hole in the workpiece, particularly in workpieces of substantial thickness. When a self-tapping fastener is introduced into the pilot hole of a workpiece, the fastener tends to have its central axis lie at an angle to the central axis of the hole of the workpiece. The reason for this lies in the fact that the tapering lead thread of the screw tends to trace a spiral path with the result that in any cross section there is substantially no more than one point from the thread whose distance from the axis is one half the pilot hole diameter. Thus, in theory the screw axis at the starting may have an angularity as much as the angle of the taper of the thread. Such angularity or "cocking" of the screw is objectionable in that there is difficulty in starting the screw in a threading operation without undue end pressure. Also the screw may end up "cocked" when fully threaded into the workpiece.